1. Field of the Invention
The present invention relates to measuring corrosion on a metallic path, and in particular to a method and apparatus for measuring the corrosion on an exposed underground neutral wire.
2. Description of the Prior Art
Many installations of padmount transformers have cables running between them that include an outer concentric neutral wire wrapped around the insulation that covers the phase conductors. This neutral wire is often a bare wire that is exposed to the earth and is subject to corrosion. Corrosion undesirably increases the resistance of the neutral wire. If the resistance becomes too high, the neutral is unable to provide a safe current return path and the cable must be replaced.
Other phenomena may also cause the neutral resistance to increase, rendering the cable unfit to use. Inadvertent damage during installation or digging and direct lightning strikes can damage the neutral strands, increasing the resistance of the neutral.
Newer installations may have a semiconducting or insulating jacket over the neutral wire to reduce corrosion. However, jacketed neutrals are also subject to damage.
At present, if an electric utility customer complains about voltage and related problems, the standard practice is to replace all suspected underground cables in the area. This practice is costly and time consuming. Furthermore, it often results in the unnecessary replacement of cables having uncorroded neutral wires. A test system that can isolate a corroded neutral wire is therefore desirable.
In residential power distribution applications, disconnecting a neutral wire from a padmount transformer to perform a test on the wire requires the crew member to interrupt the electrical service to the customers receiving power from the padmount. Interrupting electrical service is time consuming, inconvenient and sometimes impossible. Therefore, a neutral wire corrosion tester should have the ability to test for corrosion of the neutral wire without requiring disconnection of the neutral wire from regular service.
Methods of testing underground neutral wire corrosion have been reported in the professional literature. In "Overground Method Pinpoints Concentric-Neutral Corrosion," by Donald K. Baver, P. E., Transmission & Distribution, p. 48, July 1989, a neutral wire corrosion detection method is described in which a high frequency voltage is applied to the neutral wire. The step volate along the surface is measured by conducting an overland survey in which surface voltage is measured at preselected probes placed in the ground. When current meets a resistance in a ground wire, it will seek an alternate path, resulting in a corresponding voltage can be measured at the ground surface. A significant disadvantage of this method, however, is that it requires the application of a non-standard frequency to the neutral and that an overland survey be performed along the entire surface over which the cable runs to detect neutral wire corrosion.
"Underground Method Locates Concentric-Neutral Corrosion," by Walter G. Axsmith, et al., Transmission & Distribution, p. 26, July 1993, describes a method similar to that of the Baver reference, except that the overland survey is conducted by a person fitted with shoe-mounted reference electrodes who walks along the path of the line being measured.
Several devices used to detect the condition of power transmission wires are also known. For example, Vernier, (U.S. Pat. No. 4,321,643), describes a system which detects shorts and opens in ground wires wherein a complete conductive loop is established between a ground conductor and a pilot wire. A sensing signal is coupled onto the loop. Voltage and current in the loop are separately sampled and analysis is performed on these samples to detect any change in resistance. However, the Vernier device does not determine wire resistance, as is necessary for measuring corrosion, but rather detects only shorts and opens in a ground line. Additionally, the neutral wire under test requires isolation from the neutral system consisting of several parallel paths.
Sherwood (U.S. Pat. No. 4,415,850) shows a system that detects shorts and opens in ground wires that comprises power conductors and a pilot wire. An AC test signal is applied to a loop composed of the ground conductor and pilot wire and a terminating impedance and is sampled by a current transformer. However, the Sherwood device does not determine wire resistance, but rather detects only shorts and opens in a ground line.
Burbank, III (U.S. Pat. No. 4,756,241) shows a hinge clamp for securing a sensor module on a power transmission line. The sensor module contains means for sensing various parameters of a power conductor, such as power factor, voltage, frequency, etc. The signals produced by the sensors are converted to digital form and transmitted to a receiver. A zero-crossing detector is used to detect the zero crossings of the current. The Burbank device does not measure resistance, but rather it measures electrical parameters on a transmission line.
Matsuno (U.S. Pat. No. 4,875,830) shows a method for measuring insulation deterioration by measuring resistance of an electric line. It incorporates a phase-correcting device that corrects any phase shift that occurs during resistance measurement. Matsuno measures insulation resistance, and does not measure conductor resistance and, therefore, does not measure corrosion. Furthermore, it does not use line frequency current.
Hauser, et al. (German Patent No. 27 35 756) shows a method for determining earth leakage direction in compensated power networks. It involves phase analysis of voltages and currents of a frequency higher than the power frequency using a reactive idle power relay. It is designed to enable faulty conductor identification. However, it does not measure wire corrosion, and it does not use line frequency current.
Walsh et al. (U.S. Pat. No. 5,101,161) shows a non-destructive energization status tester for electric power cables that uses AC signals and resistance measurements. Carverick, et al. (U.S. Pat. No. 5,301,121) disclose a method for measuring electrical parameters of a power line operation using a digital computer. The system includes analog to digital converters for digitizing the sensor responses. Neither measures corrosion on a wire.
There is thus a need for a neutral wire corrosion tester that will enable a crew member to determine the corrosion on an underground wire without performing an overland survey, so that an objective decision as to replacement of the cable can be made.
There is also a need for a neutral wire corrosion tester that will enable a crew member to determine the corrosion on an underground wire without disconnecting electrical service to the residential customer.
There is also a need for a neutral wire corrosion tester that will enable a crew member to determine the corrosion on an underground wire using current of the same frequency as the standard electrical service frequency.